cgroup/cpu: Fix tg_has_rt_task() malfunction

Hi,
I am a graduate student of System Software Lab. at Konkuk University
(http://sslab.konkuk.ac.kr).

This patch is to fix the malfunction of tg_has_rt_tasks(). The tg_has_rt_tasks() function is supposed to determine whether a task group includes an "rt task" or not. However, it returns true for "dl task" too. The origin of this problem is that rt_task() called by tg_has_rt_tasks() returns true for dl tasks. Because of this problem, tg_rt_schedulable() that calls tg_has_rt_tasks() returns –EBUSY if we try to initialize an rt task group, while a dl task is running. That is, you cannot run an rt task group when there exists a dl task in the system. Our patch provided in this post simply makes the tg_has_rt_tasks() function return false for dl tasks by calling dl_task() in the conditional statement.

Previously we have provided another patch for rt_task() (more precisely rt_prio()) to resolve this problem as follows:
https://patchwork.kernel.org/patch/9299267/
However, a dependency issue were raised. So, we think that it is better to modify tg_has_rt_tasks() instead of rt_task() itself.

----
kernel/sched/core.c | 2 +-

 1 file changed, 1 insertion(+), 1 deletion(-)

index 2a906f2..ee5ab2f 100644
@@ -7846,7 +7846,7 @@ static inline int tg_has_rt_tasks(struct task_group *tg)
  return 0;
 
  for_each_process_thread(g, p) {
- if (rt_task(p) && task_group(p) == tg)
+ if (rt_task(p) && !dl_task(p) && task_group(p) == tg)
  return 1;
  }
 

Can you please repost with scheduler maintainers cc'd?

Ingo Molnar <[email protected]>
Peter Zijlstra <[email protected]>

Thanks.

tejun